Phobos Samples May Reveal If Mars Moons Are Captured Asteroids or Impact Debris
Japan is launching a spacecraft this week on a mission to bring back soil from Phobos, the larger of the two moons orbiting Mars. This Martian Moons Exploration mission, or MMX, marks the first time any space agency has attempted to return a sample from the Mars system to Earth.
The mission aims to resolve a long-standing debate regarding the origin of the two moons. Scientists have proposed two competing theories: either Phobos and its smaller sibling Deimos are asteroids captured by the gravity of Mars, or they are remnants of debris flung into orbit after a large object collided with the Red Planet.
According to the Japan Aerospace Exploration Agency (JAXA), the answers to these questions are hidden within the physical composition of the moon. While Phobos and Deimos appear similar to asteroids found in the outer main asteroid belt in visible light, their orbital paths suggest a different history. Both moons follow nearly circular orbits aligned with the equator of Mars, a configuration that matches the expected pattern of debris from a giant impact.
Phobos is a lumpy, irregular body measuring approximately 17 by 14 by 11 miles. Because it orbits Mars at a distance of only 6,000 kilometers three times a day, it is positioned to act as a collector of material from the planet itself. Researchers estimate that about 0.1 percent of the material collected from the surface of Phobos will consist of Martian ejecta. By analyzing this soil, scientists hope to determine how the Martian moons formed and gain a clearer understanding of the evolution of the Mars sphere, including the history of its atmosphere and the presence of liquid water on its surface.
The MMX spacecraft is scheduled to arrive at the Mars region in August 2027. It will spend time surveying both moons, as the team believes Phobos and Deimos share similar roots. The mission includes specific plans to fly by Deimos to collect observations. After arriving at Phobos, the spacecraft will enter a quasi-satellite orbit, appearing to orbit the moon while actually following a path around Mars.
The landing on Phobos will be a brief event. The spacecraft will remain on the surface for approximately two and a half hours. To collect the sample, a device developed by Honeybee Robotics will fire ultrapure nitrogen gas at the surface to blow loose soil into a container. This process acts like a vacuum cleaner to gather the material. NASA provided one of the two soil samplers used for this task, along with the Megane instrument.
Megane, which means “eyeglasses” in Japanese, is designed to perform the first chemical element measurements of Phobos. The instrument uses gamma rays and neutrons to analyze the surface composition. According to Kiyoshi Kuramoto, the principal investigator for the mission, the analysis of these soil samples will likely provide a definitive conclusion regarding the origin of the moons.
The mission also carries broader implications for planetary science. The team believes the origins of the moons are linked to the history of a Mars that once possessed a thick atmosphere and surface water. By studying the carbonaceous material on Phobos, which is similar to asteroids that may have delivered water and organic ingredients to terrestrial planets, researchers hope to uncover details about the early solar system.
Reaching the Martian moons has historically been a challenge. The Soviet Union lost two probes in 1988 and 1989 before they could reach Phobos, and a later Russian attempt in 2011, the Phobos-Grunt mission, failed to leave Earth orbit. JAXA describes MMX as the largest international collaborative mission in lunar and planetary exploration ever led by Japan. The mission involves significant contributions from international partners, including the French National Centre for Space Studies and the German Aerospace Center, which provided a 55-pound rover named IDEFIX. The European Space Agency is also providing operational and scientific support.
NASA, which contributed approximately $87 million to the mission over 14 years, will retain at least 15 percent of the returned specimens. Tom Statler, a NASA program scientist, noted that the mission is significant because it will be the first time a spacecraft returns to Earth from the Mars system. Project manager Yasuhiro Kawakatsu has described the Martian moons as the most important unclimbed mountains in the solar system, noting that they have never been fully investigated despite their potential importance.
The spacecraft is scheduled to return to Earth in 2031. Beyond the immediate goal of determining the origin of the moons, the data collected will be used to evaluate the potential of using a Martian moon as a base for future human exploration of the planet. As global space agencies look toward the prospect of sending astronauts to Mars, the information gathered by MMX regarding the environment and composition of Phobos could provide essential data for future planning.
Proximity Enables Martian Material Collection
Phobos orbits Mars at a distance of only 6,000 kilometers, completing three revolutions around the planet each day. This close proximity allows the moon to act as a natural collector of material ejected from Mars’ surface over time.
Researchers estimate that approximately 0.1 percent of the regolith gathered from Phobos will consist of Martian ejecta—rock and dust blasted into space by ancient impacts on the Red Planet.
By analyzing this small fraction of foreign material within the returned samples, scientists aim to uncover details about Mars’ geological history, including the evolution of its atmosphere and the past presence of liquid water on its surface.
Sources
- JAXA's Martian Moons Exploration mission launch: how to watch live
- Japan prepares to launch sample return mission to Phobos
- Japan to launch world-first mission to collect samples from Martian moon
Photo: NASA Hubble Space Telescope / Unsplash (Unsplash License)
